1. Field of the Invention
The present invention relates to a switch for opening and/or closing an electrical circuit, having a temperature-dependent switching mechanism and a housing, receiving the switching mechanism, which has an electrically conductive lower part as well as an electrically conductive cover part, closing off the latter, that is electrically insulated from the lower part, such that the switching mechanism, as a function of its temperature, creates an electrical connection between the cover part and the lower part, and the electrical circuit can be connected on the one hand to the cover part and on the other hand to the lower part.
2. Description of Related Art
A switch of this kind is known from DE 29 17 482 C2.
In the case of the known switch a metal housing is provided, the lower part being insulated from the cover part by interposition of an insulating film. The cover part is mounted in lossproof fashion onto the lower part by means of a crimped rim of the latter. Provided in the interior of the housing is a temperature-dependent bimetallic switching mechanism which comprises a spring disk that carries a movable contact element. A bimetallic snap disk is slipped over the contact element.
Below the response temperature of the bimetallic snap disk, the spring disk presses the movable contact element against a projection provided on the inside of the cover part, and on the other hand is braced at its rim against the inside of the lower part. Since the spring disk is made of electrically conductive material, an electrical connection is thus created between the cover part and the lower part.
If the temperature of the switch is then raised, the bimetallic snap disk then snaps over, is now braced at its rim against the inside of the cover part, and pushes the movable contact element away from the cover part against the force of the spring disk. Since the insulating film covers a large part of the inside of the cover part, the rim of the bimetallic disk, now being braced against the cover part, is insulated with respect to the cover part, so that after the contact element lifts away from the projection the electrically conductive connection between the cover part and the lower part is interrupted.
Switches of this kind are connected in series with an electrical load in an electrical circuit, provision being made for a good thermal connection between the electrical switch and the load being protected. As a result of the functional principle of the bimetallic switching mechanism as described above, the load is supplied with power as long as its temperature is low enough that the response temperature of the bimetallic snap disk is not reached. If the temperature of the load rises above a permissible value because of an operational malfunction, the electrical circuit is interrupted and the load is thus deactivated for protection from overtemperature.
The known switch with its encapsulated metal housing is very robust and insensitive to mechanical influences, so that it satisfactorily meets the demands made upon it.
A disadvantage of this switch, however, is the fact that it automatically reactivates when the load cools down, so that repeated activation and deactivation of the load occurs if the malfunction is not corrected after the first deactivation. Cycling switching behavior of this kind is, however, often undesirable.
In order to remedy this drawback, a further switch that comprises a lower part which is closed off by a cover part made of thermistor material and in which the switching material is arranged, is known from EP 0 2 84 916 A2. The bimetallic switching mechanism comprises, in known fashion, a bimetallic snap disk as well as a spring disk on which a movable contact element is held. Below the response temperature of the bimetallic snap disk, the movable contact element is pressed by the spring disk against a fixed contact element that is provided on the cover part, extends through the cover part in the manner of a rivet, and transitions externally into a head. The lower part is made of electrically conductive material, so that at low temperatures a conductive connection is created between the lower part and the head of the fixed contact element. The cover part is conductively connected both to the fixed contact element and to the lower part, so that it is connected electrically in parallel with the switching mechanism.
When the switching mechanism then opens as a result of excessive temperature, current thus flows from the fixed contact element, through the PTC thermistor constituting the cover part, to the lower part, thus causing the PTC thermistor to heat up and hold the switching mechanism open, even if the overtemperature triggering the switching action is no longer present. The PTC thermistor thus acts to provide a self-holding function.
In a further embodiment from this document, the cover part comprises a ceramic support part on which is arranged a carbon resistor which, as a heating resistor, provides the self-hold function.
If the cover part is made of thermistor material, it does not have the requisite compression stability often required by the known switches in rough everyday use. Switches of this kind are used for temperature monitoring of motors, heating coils, etc., so they are often exposed to severe mechanical stresses as a result of the vibrations associated with operation of the loads being protected. Severe pressures can also be exerted on the cover of the temperature controller.
If the parallel resistor is a carbon resistor, the cover itself can be made of a mechanically more stable material, but just as with the cover part made of thermistor material, a through contact outward through the cover is necessary, which is not required in the case of the switch discussed at the outset.
The switch known from EP 0 284 916 A2 thus has the advantage over the switch mentioned at the outset that it is equipped with a self-hold function, but on the other hand has other disadvantages consisting of complex design and reduced mechanical strength.
Lastly, DE 43 36 564 A1 discloses a further self-holding switch having a PTC thermistor connected in parallel, a further heating resistor being connected in series with the switching mechanism and providing overcurrent sensitivity for the known switch.
This switch comprises a ceramic support plate, equipped with conducting and insulating coatings, on which is arranged an encapsulated bimetallic switching mechanism next to which sits the thermistor module, which is connected electrically in parallel with the switching mechanism. Also arranged on the ceramic support plate is a thick-film resistor that passes beneath the switching mechanism and is connected in series with it.
The known switch is also connected in series with a load being protected, so that the operating current of that load flows through it. At the same time this switch is thermally connected, in a known manner, with the load being protected. If the operating current of the load increases impermissibly due to a defect, the thick-film resistor, connected in series, heats up the switching mechanism to the extent that it opens, so that the PTC thermistor, connected in parallel, accepts the current. Because of the high resistance of the PTC thermistor, the operating current of the load then decreases to a harmless level which is nevertheless sufficient, by way of the ohmic loss in the PTC thermistor, to maintain a temperature which holds the switching mechanism open.
Of course this switch will also open when the temperature of the load being protected is too high; here again, the PTC thermistor provides for self-holding of the switching mechanism, which is now open.
A disadvantage of this switch is that its construction is relatively cumbersome and large, a fact attributable in particular to the ceramic support plate.
More stringent safety requirements as well as new safety regulations make it necessary for the switch mentioned initially, which is often also referred to as a temperature controller, to be equipped with a self-hold function and/or with overcurrent sensitivity. The known switches described above, however, which have such functions, are not satisfactory in terms of mechanism and construction, their high production costs being particularly disadvantageous.